Membrane probing system with local contact scrub

ABSTRACT

A membrane probing assembly includes a support element having an incompressible forward support tiltably coupled to a rearward base and a membrane assembly, formed of polyimide layers, with its central region interconnected to the support by an elastomeric layer. Flexible traces form data/signal lines to contacts on the central region. Each contact comprises a rigid beam and a bump located in off-centered location on the beam, which bump includes a contacting portion. After initial touchdown of these contacting portions, further over-travel of the pads causes each beam to independently tilt locally so that different portions of each beam move different distances relative to the support thus driving each contact into lateral scrubbing movement across the pad thereby clearing away oxide buildup. The elastomeric member backed by the incompressible support ensures sufficient scrub pressure and reliable tilt recovery of each contact without mechanical straining of the beam. In an alternative embodiment, the contacts comprise conductive beams each supported on a loose U-shaped flap formed in the membrane assembly where each flap and beam is tiltably supported in inclined position by an elastomeric hub interposed between the flap and support.

BACKGROUND OF THE INVENTION

The present invention relates to probe assemblies of the type commonlyused for testing the individual devices that comprise an integratedcircuit (IC) and, in particular, the present invention relates to amembrane probing assembly having contacts which scrub, in a locallycontrolled manner, across the respective input/output conductors of eachdevice so as to reliably wipe clear the surface oxides that are normallyfound on those conductors thereby ensuring good electrical connectionbetween the probing assembly and each device.

The trend in electronic production has been toward increasingly smallergeometries particularly in integrated circuit technology wherein a verylarge number of discrete circuit elements are fabricated on a singlesubstrate or “wafer.” After fabrication, this wafer is divided into anumber of rectangular-shaped chips or “dies” where each die presents arectangular or other regular arrangement of metallized contact padsthrough which input/output connections are made. Although each die iseventually packaged separately, for efficiency sake, testing of thecircuit formed on each die is preferably performed while the dies arestill joined together on the wafer. One typical procedure is to supportthe wafer on a flat stage or “chuck” and to move the wafer in X, Y and Zdirections relative to the head of the probing assembly so that thecontacts on the probing assembly move from die to die for consecutiveengagement with each die. Respective signal, power and ground lines arerun to the probing assembly from the test instrumentation thus enablingeach circuit to be sequentially connected to the test instrumentation.

One conventional type of probing assembly used for testing integratedcircuits provides contacts that are configured as needle-like tips Thesetips are mounted about a central opening formed in a probe card so as toradially converge inwardly and downwardly through the opening. When thewafer is raised beyond that point where the pads on the wafer first comeinto contact with these tips, the tips flex upwardly so as to skateforwardly across their respective pads thereby removing oxide buildup onthe pads.

The problem with this type of probing assembly is that the needle-liketips, due to their narrow geometry, exhibit high inductance so thatelectrical losses are high in measurements made through these tips.Also, these tips can act in the manner of a planing tool as they wipeacross their respective pads, thereby leading to excessive pad wear.This problem is magnified to the extent that the probe tips bend out ofshape during use or otherwise fail to terminate in a common plane whichcauses the more forward ones of the tips to bear down too heavily ontheir respective pads Also, it is impractical to mount these tips atless than 100 micron center-to-center spacing or in a multi-rowgrid-like pattern so as to accommodate the pad arrangement of moremodern, higher density dies.

In order to reduce inductive losses, decrease pad wear and accommodatesmaller device geometries, a second type of probing assembly has beendeveloped that uses a flexible membrane structure for supporting theprobing contacts. In this assembly, lead lines of well-defined geometryare formed on one or more plies of flexible insulative film, such aspolyimide or MYLAR™. If separate plies are used, these plies are bondedtogether to form, for example, a multilayered transmission linestructure. In the central portion of this flexible structure ormembrane, each conductive line is terminated by a respective probingcontact which is formed on, and projects outwardly from, an outer faceof the membrane. These probing contacts are arranged in a predeterminedpattern that matches the pattern of the device pads and typically areformed as upraised bumps for probing the flat surfaces conventionallydefined by the pads. The inner face of the membrane is supported on asupporting structure. This structure can take the form, for example, ofa truncated pyramid, in which case the inner face of the center portionof the membrane is supported on the truncated end of the support whilethe marginal portions of the membrane are drawn away from the centerportion at an angle thereto so as to clear any upright components thatmay surround the pads on the device.

With respect to the membrane probing assembly just described, excessiveline inductance is eliminated by carefully selecting the geometry of thelead lines, and a photolithographic process is preferably used to enableprecise control over the size, spacing, and arrangement, of the probingcontacts so as to accommodate higher density configurations. However,although several different forms of this probing assembly have beenproposed, difficulties have been encountered in connection with thistype of assembly in reducing pad wear and in achieving reliable clearingof the oxide layer from each of the device pads so as to ensure adequateelectrical connection between the assembly and the device-under-test.

One conventional form of membrane probing assembly, for example, isexemplified by the device shown in Rath European Patent Pub. No.259,163A2. This device has the central portion of the sheet-likemembrane mounted directly against a rigid support. This rigid support,in turn, is connected by a resilient member comprising an elastomeric orrubber block to the main body of the assembly so that the membrane cantilt to match the tilt of the device. Huff U.S. Pat. No. 4,918,383 showsa closely related device wherein radially extending leaf springs permitvertical axis movement of the rigid support while preventing it fromtilting so that there is no slippage or “misalignment” of the contactbumps on the pads and further so that the entire membrane will shiftslightly in the horizontal plane to allow the contacts to “scrub” acrosstheir respective pads in order to clear surface oxides from these pads.

In respect to both of these devices, however, because of manufacturingtolerances, certain of the contact bumps are likely to be in a recessedposition relative to their neighbors and these recessed bumps will nothave a satisfactory opportunity to engage their pads since they will bedrawn away from their pads by the action of their neighbors on the rigidsupport. Furthermore, even when “scrub” movement is provided in themanner of Huff, the contacts will tend to frictionally cling to thedevice as they perform the scrubbing movement, that is, there will be atendency for the pads of the device to move in unison with the contactsso as to negate the effect of the contact movement. Whether anyscrubbing action actually occurs depends on how far the pads can move,which depends, in turn, on the degree of lateral play that exists as aresult of normal tolerance between the respective bearing surfaces ofthe probe head and chuck. Hence this form of membrane probing assemblydoes not ensure reliable electrical connection between each contact andpad.

A second conventional form of membrane probing assembly is exemplifiedby the device shown in Barsotti European Patent Pub. No. 304,868A2. Thisdevice provides a flexible backing for the central or contact-carryingportion of the flexible membrane. In Barsotti, the membrane is directlybacked by an elastomeric member and this member, in turn, is backed by arigid support so that minor height variations between the contacts orpads can be accommodated. It is also possible to use positive-pressureair, negative-pressure air, liquid or an unbacked elastomer to provideflexible backing for the membrane, as shown in Gangroth U.S. Pat. No.4,649,339, Ardezzone U.S. Pat. No. 4,636,772, Reed, Jr. et al. U.S. Pat.No. 3,596,228 and Okubo et al. U.S. Pat. No. 5,134,365, respectively.These alternative devices, however, do not afford sufficient pressurebetween the probing contacts and the device pads to reliably penetratethe oxides that form on the pad surfaces.

In this second form of membrane probing assembly, as indicated in Okubo,the contacts may be limited to movement along the Z-axis in order toprevent slippage and resulting misalignment between the contacts andpads during engagement. Thus, in Barsotti, the rigid support underlyingthe elastomeric member is fixed in position although it is also possibleto mount the support for Z-axis movement in the manner shown in HuffU.S. Pat. No. 4,980,637. Pad damage is likely to occur with this type ofdesign, however, because a certain amount of tilt is typically presentbetween the contacts and the device, and those contacts angled closestto the device will ordinarily develop much higher contact pressures thanthose which are angled away. The same problem arises with the relatedassembly shown in European Patent Pub. No. 230,348A2 to Garretson, eventhough in the Garretson device the characteristic of the elastomericmember is such as to urge the contacts into lateral movement when thosecontacts are placed into pressing engagement with their pads. Yetanother related assembly is shown in Evans U.S. Pat. No. 4,975,638 whichuses a pivotably mounted support for backing the elastomeric member soas to accommodate tilt between the contacts and the device. However, theEvans device is subject to the friction clinging problem alreadydescribed insofar as the pads of the device are likely to cling to thecontacts as the support pivots and causes the contacts to shiftlaterally.

Yet other forms of conventional membrane probing assemblies are shown inCrumly U.S. Pat. No. 5,395,253, Barsotti et al. U.S. Pat. No. 5,059,898and Evans et al. U.S. Pat. No. 4,975,638. In Crumly, the center portionof a stretchable membrane is resiliently biased to a fully stretchedcondition using a spring. When the contacts engage their respectivepads, the stretched center portion retracts against the spring to apartially relaxed condition so as to draw the contacts in radial scrubdirections toward the center of the membrane. In Barsotti, each row ofcontacts is supported by the end of a respective L-shaped arm so thatwhen the contacts in a row engage their respective pads, thecorresponding arm flexes upwardly and causes the row of contacts tolaterally scrub simultaneously across their respective pads. In bothCrumly and Barsotti, however, if any tilt is present between thecontacts and the device at the time of engagement, this tilt will causethe contacts angled closet to the device to scrub further than thoseangled further away. Moreover, the shorter contacts will be forced tomove in their scrub directions before they have had the opportunity toengage their respective pads due to the controlling scrub action oftheir neighboring contacts. A further disadvantage of the Crumly device,in particular, is that the contacts nearer to the center of the membranewill scrub less than those nearer to the periphery so that scrubeffectiveness will vary with contact position.

In Evans et al. U.S. Pat. No. 5,355,079 each contact constitutes aspring metal finger, and each finger is mounted so as to extend in acantilevered manner away from the underlying membrane at a predeterminedangle relative to the membrane. A similar configuration is shown inHiggins U.S. Pat. No. 5,521,518. It is difficult, however, to originallyposition these fingers so that they all terminate in a common plane,particularly if a high density pattern is required. Moreover, thesefingers are easily bent out of position during use and cannot easily berebent back to their original position. Hence, certain ones of thefingers are likely to touch down before other ones of the fingers, andscrub pressures and distances are likely to be different for differentfingers. Nor, in Evans at least, is there an adequate mechanism fortolerating a minor degree of tilt between the fingers and pads. AlthoughEvans suggests roughening the surface of each finger to improve thequality of electrical connection, this roughening can cause undueabrasion and damage to the pad surfaces. Yet a further disadvantage ofthe contact fingers shown in both Evans and Higgins is that such fingersare subject to fatigue and failure after a relatively low number of“touchdowns” or duty cycles due to repeated bending and stressing.

In accordance with the foregoing, an object of at least preferredembodiments of the present invention is to provide a probing assemblywhich can reliably make electrical connection with a high densityarrangement of pads on an electrical device despite the buildup ofoxides or other contaminants on the pad surfaces.

Another object of at least preferred embodiments of the presentinvention is to provide a membrane probing assembly that ensuresadequate scrubbing action between each contact and pad despite minorvariations in contact height.

Another object of at least preferred embodiments of the presentinvention is to provide a probing assembly that can reliably operateover a large number of duty cycles without failure.

Another object of at least preferred embodiments of the presentinvention is to provide a membrane probing assembly providing uniformlyeffective scrubbing action in respect to the contacts independent of therespective positions of the contacts.

SUMMARY OF THE INVENTION

A probing assembly system for probing an electrical device in accordancewith the present invention provides a forward support of incompressiblematerial, a flexible membrane assembly mounted with its central regionoverlying the support, a plurality of rigid contacts disposed on themembrane assembly within the central region each including a beam and acontacting portion, each contacting portion being arranged in suitableposition for pressing engagement with a corresponding pad on the device,each beam being electrically connected to a corresponding flexibleconductor that extends into the central region. In accordance with thepresent invention, the probing assembly further provides a pressurecontrol mechanism including an elastomeric member interposed betweeneach contact and the support and a motion control mechanism locallyoperating in respect to each contact to urge each beam, when thecorresponding contacting portion is placed into pressing engagement withthe respective pad, into tilting motion so that different portions ofeach beam move different distances relative to the forward support andso that each contact is driven, in accordance with this tilting motion,into a lateral scrubbing movement across the corresponding pad. Theelastomeric member is positioned so as to resiliently oppose the tiltingmotion and so as to enable each contact to recover from this tiltingmotion in a manner avoiding mechanical straining of each beam.

In accordance with the foregoing system, a high-density arrangement ofcontacts can be provided on the probing assembly using, for example, aphotolithographic process to define both the contacts and their lead-inconductors on the flexible membrane assembly. The resiliency of theelastomeric member accommodates minor height variations that may existbetween adjacent ones of the contacts, and the incompressible supportthat backs the elastomeric member ensures that this member willsufficiently bear against the contacts to maintain an adequate level ofcontact-to-pad pressure during scrubbing. The elastomeric member furtherserves as a reliable mechanism for enabling the beam of each rigidcontact to recover after tilting thereby avoiding the need to rely onthe type of elastic recovery provided by mechanically strained contactneedles or fingers, which alternative system can lead to ineffectiveelectrical performance as the contact needles or fingers bend out ofshape during use or prematurely fail due to bending fatigue. Further inaccordance with the present invention, by urging each contact intotilting motion by means of a locally operating motion control mechanismso that different portions of each beam move different distancesrelative to the support and so that each contact is thereby driven intolateral wiping movement across the corresponding pad, scrubbing actionbetween the contacts and pads is achieved in a manner effective forclearing oxides from the pads. It is possible, in particular, to achievea high level of scrub effectiveness in relation to each contact on theassembly in accordance with this type of localized tilt control.

The foregoing and other objectives, features, and advantages of theinvention will be more readily understood upon consideration of thefollowing detailed description of the invention, taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing an exemplary membrane probingassembly bolted to a probe head and a wafer supported on a chuck insuitable position for probing by this assembly.

FIG. 2 is a bottom elevational view showing various parts of the probingassembly of FIG. 1, including a support element and flexible membraneassembly, and a fragmentary view of a probe card having data/signallines connected with corresponding lines on the membrane assembly.

FIG. 3 is a side elevational view of the membrane probing assembly ofFIG. 1 where a portion of the membrane assembly has been cut away toexpose hidden portions of the support element.

FIG. 4 is a top elevational view of an exemplary support element.

FIGS. 5 a-5 b are schematic side elevational views illustrating how theexemplary support element and membrane assembly are capable of tiltingto match the orientation of the device under test.

FIG. 6 is an enlarged top elevational view of the central region of anexemplary construction of the membrane assembly of FIG. 2.

FIGS. 7 a-7 b are sectional views taken along lines 7 a-7 a in FIG. 6first showing an exemplary contact before touchdown and then showing thesame contact after touchdown and scrub movement across its respectivepad.

FIG. 8 is a schematic side view showing, in dashed-line representation,the exemplary contact of FIGS. 7 a-7 b at the moment of initialtouchdown and, in solid-line representation, the same contact afterfurther vertical overtravel by the pad.

FIGS. 9 a-9 b are schematic side views showing three of the exemplarycontacts of FIG. 6 and how such contacts engage their respective padsdespite minor height variations between the contacts.

FIGS. 10 a-10 b are top views comparable to FIG. 6 but showingalternative contact patterns. FIG. 10 c is a side elevational viewshowing the appearance of the contacts in each of these patterns aftertouchdown and scrub across their respective contacts.

FIGS. 11 a-11 f are sectional side views of an alternative embodiment ofthe center portion of the membrane probing assembly in successive stagesof manufacture. FIG. 11 f shows the final construction.

FIG. 12 is a schematic side view showing, in dashed-line representation,a contact of the alternative embodiment of FIG. 11 f at the moment ofinitial touchdown and, in solid-line representation, the same contactafter further vertical overtravel by the pad.

FIG. 13 is a top elevational view of the alternative embodiment shown inFIG. 11 f.

FIG. 14 is a top elevational view of a modified form of the alternativeembodiment shown in FIG. 11 f.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a probe head 40 for mounting a membrane probing assembly 42constructed in accordance with the present invention. In order tomeasure the electrical performance of a particular die area 44 includedon the silicon wafer 46, the high-speed digital lines 48 and/or shieldedtransmission lines 50 of the probe head are connected to theinput/output ports of the test instrumentation by a suitable cableassembly, and the chuck 51 which supports the wafer is moved in mutuallyperpendicular X,Y,Z directions in order to bring the pads of the diearea into pressing engagement with the contacts included on the lowercontacting portion of the membrane probing assembly.

The probe head 40 includes a probe card 52 on which the data/signallines 48 and 50 are arranged. Referring to FIGS. 2-3, the membraneprobing assembly 42 includes a support element 54 formed ofincompressible material such as a hard polymer. This element isdetachably connected to the upper side of the probe card by means offour Allen screws 56 and corresponding nuts 58 (each screw passesthrough a respective attachment arm 60 of the support element, and aseparate backing element 62 evenly distributes the clamping pressure ofthe screws over the entire back side of the supporting element). Inaccordance with this detachable connection, different probing assemblieshaving different contact arrangements can be quickly substituted foreach other as needed for probing different devices.

Referring to FIGS. 3-4, the support element 54 includes a rearward baseportion 64 to which the attachment arms 60 are integrally joined. Alsoincluded on the support element 54 is a forward support or plunger 66that projects outwardly from the flat base portion. This forward supporthas angled sides 68 that converge toward a flat support surface 70 so asto give the forward support the shape of a truncated pyramid. Referringalso to FIG. 2, a flexible membrane assembly 72 is attached to thesupport after being aligned by means of alignment pins 74 included onthe base portion. This flexible membrane assembly is formed by one ormore plies of insulative sheeting such as KAPTON™ sold by E.I. Du Pontde Nemours or other polyimide film, and flexible conductive layers orstrips are provided between or on these plies to form the data/signallines 76.

When the support element 54 is mounted on the upper side of the probecard 52 as shown in FIG. 3, the forward support 66 protrudes through acentral opening 78 in the probe card so as to present the contacts whichare arranged on a central region 80 of the flexible membrane assembly insuitable position for pressing engagement with the pads of the deviceunder test. Referring to FIG. 2, the membrane assembly includes radiallyextending arm segments 82 that are separated by inwardly curving edges84 that give the assembly the shape of a formee cross, and thesesegments extend in an inclined manner along the angled sides 68 therebyclearing any upright components surrounding the pads. A series ofcontact pads 86 terminate the data/signal lines 76 so that when thesupport element is mounted, these pads electrically engage correspondingtermination pads provided on the upper side of the probe card so thatthe data/signal lines 48 and 50 on the probe card are electricallyconnected to the contacts on the central region.

An important feature of the exemplary membrane probing assembly 42 isits capability for probing a high-density arrangement of contact padsover a large number of contact cycles in a manner that ensures reliableelectrical connection between the contacts and pads in each cycledespite oxide buildup on the pads. This capability is a function of theconstruction of the support element 54, the flexible membrane assembly72 and their manner of interconnection. In particular, the membraneassembly is so constructed and connected to the support element that thecontacts on the membrane assembly preferably wipe or scrub, in a locallycontrolled manner, laterally across the pads when brought into pressingengagement with these pads. The preferred mechanism for producing thisscrubbing action will now be described in connection with theconstruction and interconnection of a preferred membrane assembly 72 aas best depicted in FIGS. 6 and 7 a-7 b.

FIG. 6 shows an enlarged view of the central region 80 a of thepreferred membrane assembly 72 a. In this embodiment, the contacts 88are arranged in a square-like pattern suitable for engagement with asquare-like arrangement of pads. Referring also to FIG. 7 a, whichrepresents a sectional view taken along lines 7 a-7 a in FIG. 6, eachcontact comprises a relatively thick rigid beam 90, preferably 150 to250 microns long, at one end of which is formed a rigid contact bump 92.The contact bump includes thereon a contacting portion 93, which, in theembodiment illustrated, comprises a nub of rhodium nickel alloy fused tothe contact bump, although the contacting portion may, alternatively,comprise the rounded sides of the contact bump itself. Using aconventional process, such as electroplating, each beam is preferablyformed in an overlapping connection with the end of a flexibleconductive trace 76 a to form a large surface joint therewith. Thisconductive trace in conjunction with a backplane conductive layer 94effectively provides a controlled impedance data/signal line to thecontact and hence its dimensions are preferably established preciselyusing a photolithographic process. The backplane layer preferablyincludes openings therein to assist, for example, with gas ventingduring fabrication.

For ease of illustration, the flexible membrane assembly 72 a is shownas having only a single dielectric ply 96, preferably polyimide;however, multiple dielectric plies and conductive layers will normallybe used in accordance with conventional layering techniques. Themembrane assembly is interconnected to the flat support surface 70 by aninterposed elastomeric layer 98, which layer is coextensive with thesupport surface and can be formed by a silicon rubber compound such asELMER'S STICK-ALL™ made by the Borden Company or Sylgard 182 by DowCorning Corporation. This compound can be conveniently applied in apaste-like phase which hardens as it sets. The flat support surface, aspreviously mentioned, is made of incompressible material and ispreferably a hard dielectric such as polysulfone or glass.

In accordance with the above-described construction, when one of thecontacts 88 is brought into pressing engagement with a respective pad100, as indicated in FIG. 7 b, the resulting off-center force on therigid beam 90 and bump 92 structure causes the beam to pivot or tiltagainst the elastic recovery force provided by the elastomeric pad 98.This tilting motion is localized in the sense that a forward portion 102of the beam moves a greater distance toward the flat support surface 70than a rearward portion 104 of the same beam. The effect is such as todrive the contact into lateral scrubbing movement across the pad as isindicated in FIG. 7 b with a dashed-line and solid-line representationshowing the beginning and ending positions, respectively, of the contacton the pad. In this fashion, the insulative oxide buildup on each pad isremoved so as to ensure reliable contact-to-pad electrical connections.

FIG. 8 shows, in dashed line view, the relative positions of the contact88 and pad 100 at the moment of initial engagement or touchdown and, insolid-line view, these same elements after “overtravel” of the pad by adistance 106 in a vertical direction directly toward the flat supportsurface 70. As indicated, the distance 106 of lateral scrubbing movementis directly dependent on the vertical deflection of the contact 88 or,equivalently, on the overtravel distance 108 moved by the pad 100.Hence, since the overtravel distance for each contact on the centralregion 80 a will be substantially the same (with differences onlyarising from minor variations in contact height), the distance oflateral scrubbing movement by each contact on the central region will besubstantially uniform and will not, in particular, be affected by therelative position of each contact on the central region.

Because the elastomeric layer 98 is backed by the incompressible supportsurface 70, the elastomeric layer exerts a sufficient recovery force oneach tilting beam 90 and thus each contact 88 to maintain an adequatelevel of contact-to-pad pressure during scrubbing. At the same time, theelastomeric layer accommodates minor height variations between therespective contacts. Thus, referring to FIG. 9 a, when a relativelyshorter contact 88 a is situated between an immediately adjacent pair ofrelatively taller contacts 88 b and these taller contacts are broughtinto engagement with their respective pads, then, as indicated in FIG. 9b, deformation by the elastomeric layer allows the smaller contact to bebrought into engagement with its pad after only a small amount offurther overtravel by the pads. It will be noted, in this example, thatthe tilting action of each contact is locally controlled, and the largercontacts are able, in particular, to tilt independently of the smallercontact so that the smaller contact is not urged into lateral movementuntil it has actually touched down on its pad.

In respect to the preferred membrane assembly 72 a, not only is thedistance and pressure of the scrub action of each contact 88 wellregulated, but so also is the scrub direction. In FIG. 6 the respectivescrub directions of contacts 88 c-f are indicated by means ofdirectional arrows 112 c-f. As shown, each contact scrubs forwardly inthe direction of the longitudinal axis of its beam 90. Preferably, asfurther shown, the contacts are arranged in pairs (88 c, 88 d or 88 e,88 f) in which the respective contacts of each pair scrub in oppositedirections. Accordingly, in relation to the wafer or device on which thepads are formed, the scrubbing force exerted on the device by the firstcontact of each pair is cancelled out by the scrubbing force exerted bythe second contact of each pair. Hence, the device is not draggedfrictionally along by these scrubbing forces in a manner that can reduceor even prevent desirable scrubbing action between the contacts and pads

In addition to providing reliable electrical connection with the deviceunder test, the preferred membrane assembly 72 a is able to provide suchconnection over a large number of contact or touchdown cycles. Theelastomeric layer 98, in particular, serves as a reliable mechanism forenabling each beam 90 to recover from each touchdown operation, therebyavoiding the need to rely on a failure prone recovery mechanism such asbending and mechanical straining of any portion of the stiff contactsthemselves. To prevent excessive wearing of the contacts 88, asmentioned above, each contact includes a contacting portion or nub 93 ofrhodium nickel alloy, and the beam 90 and contact bump 92 are alsoformed of nickel alloy. The flexible traces 76 a are preferably ofhighly conductive metal, such as gold, for good connection with thecontacts.

Referring to FIG. 4, L-shaped slots 114 are formed in the rearward baseportion 64 to permit the forward support 66 to tilt relative to the baseportion. Accordingly, if the flat support surface 70 and the deviceunder test 116 are in tilted relationship to each other prior totouchdown, as shown in FIG. 5 a, when touchdown occurs, the engagementpressure between the contacts 88 and pads 100 will automatically movethe support surface to a position parallel with the device, as shown inFIG. 5 b. This prevents those ones of the contacts that are initiallyangled closest to the device from bearing down too heavily on their padswhich can result in damage to the pad surfaces. Also, this normalizesthe distance between the support surface and each pad once fullengagement is reached. Since this pad-to-surface distance determines theamount of deflection of each contact and since this deflectiondetermines the scrub pressure and distance for each contact asherein-above described, the respective scrub pressures and distances ofthe contacts are therefor normalized in respect to each other despiteany ordinary amount of initial tilt that may exist between the supportsurface and device.

In addition to providing reliable electrical connections over a largenumber of contact cycles, the exemplary membrane probing assembly 42 canalso be configured to accommodate the high density arrangement ofcontact pads of the type found on more modern devices. Referring, forexample, to FIG. 10 a, the contacts 88 can be arranged so as to bealigned with each other in rows and columns. The contact bumps 92 thusconform to a 2-dimensional grid pattern corresponding to the pad patternof higher density devices. A 2-dimensional grid pattern can also beachieved by arranging the contacts in chevron-shaped pairs as shown inFIG. 10 b. In this arrangement, the length of each beam 90 is preferablylonger than or nearly as long as half the spacing between the contactbumps so that only a one-dimensional deformation of the polyimide plylayers 116 is required as schematically represented in FIG. 10 c (thedeformation of the polyimide ply layers for the row and columnarrangement of contacts would look the same from the view shown) Inthese higher density embodiments, the conductive traces that form thedata/signal lines to the contacts pass between the polyimide layers, andeach trace is connected to the corresponding contact by a conductivethrough via (not shown).

An exemplary flexible membrane assembly 72 a and its interconnection tothe flat support surface 70 by an elastomeric layer 98 has now beendescribed. This construction provides an exemplary embodiment for thecontacting portion of the membrane probing assembly 42. However, it willbe recognized that alternative embodiments of the contacting portion arealso possible without departing from the broader principles of thepresent invention.

FIGS. 11 a-11 e show a sequence of steps used in the construction of analternative embodiment 174 of the center portion of the membraneassembly. The finished construction is shown in FIGS. 1 f and 13.

Referring to FIG. 11 f, in this alternative embodiment, rigid contactbeams 176, such as of rhodium nickel alloy, are provided on the outerface of the flexible membrane assembly 178. To accommodate a highdensity contact pattern (FIG. 13), the membrane assembly is formed bymultiple polyimide layers 180 (only two layers are shown for ease ofillustration) and includes a conductive backplane 182 and flexiblestriplike lines 184 that pass through the layers. Each line connects toa respective contact beam by a through via 186 and a short conductivetrace 188 (preferably of highly conductive metal such as gold for goodelectrical contact with the nickel alloy beam) to provide the requireddata/signal line.

A bonding compound 190 attaches the membrane assembly to the flatsupport surface 70 except for U-shaped flap portions 192 of theassembly. These flap portions each support a respective one of thecontact beams and are formed by U-shaped slits 194 (FIG. 13) cut throughthe assembly. An elastomeric hub 196 is attached beneath each flapportion. This hub maintains the corresponding contact beam in aninclined position relative to the support surface and enables the beamto tilt in a localized manner in relation to the surface.

Referring to FIG. 12, when each contact beam 176 touches down on itsrespective pad 100, the U-shaped flap portion 192 is deflected towardthe flat support surface 70 against the resilient bias provided by theelastomeric hub 196. This causes the contact beam to tilt relative tothe support surface in such a manner that different portions (forwardand rearward) of this beam-shaped contact move different distancesrelative to the surface and the contact beam is driven into lateralscrubbing movement across the pad The distance 198 of this lateralscrubbing movement directly depends on the distance of verticaldeflection of the contact beam or, equivalently, on the distance ofovertravel 200 of the pad beyond the position of initial touchdown.Aside from minor variations in contact height, this distance will besubstantially the same for each beam-shaped contact thereby resulting inuniform scrub movements by the different contacts.

Although they are not as convenient to manufacture as the unitaryelastomeric layer 98 (refer to FIG. 8), the elastomeric hubs 196collectively provide the same primary functions as this layer. That is,each elastomeric hub, acting in conjunction with the incompressiblesupport surface 70, ensures that the corresponding contact beam 176 willbear down with sufficient pressure as it scrubs across its pad and, onceengagement is complete, ensures that the contact beam will recover fromits tilting motion in a manner that avoids bending or mechanicalstraining of the beam. This approach is preferable over that ofproviding a contact beam or finger where a portion of the beam is bentin a cantilevered manner away from the underlying support. Under thislatter approach, the inherent resiliency of the metal contained in thebeam is relied on to provide the recovery mechanism, and the beam istherefor subject to strain fatigue and premature failure.

Referring to FIGS. 11 a and 11 b, in constructing the alternativecontacting portion 174, first a sacrificial substrate 204 is providedand openings 206 are formed in the substrate, nominally of 3 mil depth,in the desired locations for the elastomeric hubs 196. The openings arethen filled with silicon rubber to form the hubs 196, as shown in FIG.11 c, and the preassembled flexible membrane assembly 178 is bonded tothe substrate and hubs, as shown in FIG. 11 d. Next the sacrificialsubstrate is dissolved or etched away leaving the hubs attached to themembrane assembly, and the U-shaped slits 194 are cut through theassembly as shown in FIG. 11 e. Finally, the bonding compound 190 isapplied to the back-side of the membrane assembly so as to run abouthalfway into each U-shaped slit, and the entire assembly is pressed downagainst the support surface 70 in such a manner that each beam-shapedcontact 176 is turned outwardly by means of rolling action about thecorresponding hub to an inclined position as shown in FIG. 11 f. Thefinished construction is shown in side sectional view in FIG. 1 f and intop view in FIG. 13.

In the contact pattern shown in FIG. 13, the contact beams 176 arearranged in parallel rows of opposite direction on the central region208 of the flexible membrane assembly Alternative contact patterns arealso possible as shown in FIG. 14 where, on the central region 210, thecontact beams 212 are arranged in clusters of four where each contact ina particular cluster extends in one of the four compass directions(north, south, east, or west). It will be noted that in neither patterndo all the contacts scrub in the same direction in a manner likely tocause the pads of the device to be dragged frictionally along by thecontacts. Hence the contact patterns shown help to ensure that effectivescrubbing action occurs between the contact beams and pads.

An exemplary construction for a membrane probing assembly 42 as well asan alternative construction for the center portion of the assembly and avariation thereof have now been shown and described. However, it will berecognized that other constructions are possible without departing fromthe broader principles of the present invention.

The terms and expressions which have been employed in the foregoingspecification are used therein as terms of description and not oflimitation, and there is no intention, in the use of such terms andexpressions, of excluding equivalents of the features shown anddescribed or portions thereof, it being recognized that the scope of theinvention is defined and limited only by the claims which follow.

1. A probing assembly for probing an electrical device comprising: (a) aforward support of incompressible material; (b) a flexible membraneassembly having a central region positioned in overlying relationship tosaid forward support; (c) a plurality of rigid contacts disposed on saidcentral region each including a beam and a contacting portion, eachcontacting portion being arranged in suitable position for pressingengagement with a corresponding pad on said device, each beam beingelectrically connected to a corresponding flexible conductor thatextends into said central region; (d) a pressure control mechanismincluding an elastomeric member interposed between each contact and saidsupport; and (e) a motion control mechanism locally operating in respectto each contact to urge each beam, when the corresponding contactingportion is placed into pressing engagement with the respective pad, intotilting motion so that different portions of each beam move differentdistances relative to said forward support and so that each contact isdriven in accordance with said tilting motion into lateral scrubbingmovement across the corresponding pad, said elastomeric member beingpositioned so as to enable each beam to recover from said tilting motionin a manner avoiding mechanical straining of each beam.
 2. The probingassembly of claim 1 wherein each one of said beams is tiltableindependently of the other ones of said beams.
 3. The probing assemblyof claim 1 including a rearward base, said forward support beingtiltably coupled to said rearward base so as to enable said forwardsupport to automatically tilt relative to said rearward base toward aposition parallel to said device in response to pressing engagementbetween respective ones of said contacting portions and correspondingones of said pads.
 4. The probing assembly of claim 1 wherein saidcontacting portion of each contact is included on a contact bump fixedlyjoined in off-centered location on the corresponding beam.
 5. Theprobing assembly of claim 1 wherein said contacts are provided in pairsand the respective lateral scrubbing movements of said contacts in eachpair are in opposite directions.
 6. The probing assembly of claim 1wherein the distance of said lateral scrubbing movement of each contactafter touchdown of each contacting portion on the corresponding pad isuniformly dependent for each contact on the distance traveled aftertouchdown in reducing the spacing between said support and saidcorresponding pad.
 7. The probing assembly of claim 1 wherein saidelastomeric member is a unitary structure.
 8. The probing assembly ofclaim 1 wherein said contacts are arranged by rows and columns.
 9. Theprobing assembly of claim 1 wherein each beam is joined in overlappingrelationship to the corresponding flexible conductor.
 10. The probingassembly of claim 1 wherein said flexible membrane assembly issubstantially continuous along said central region.
 11. The probingassembly of claim 1 wherein each contact is supported by a continuousportion of said flexible membrane assembly.
 12. A method for probing anelectrical device comprising: (a) providing a forward support ofincompressible material; (b) providing a flexible membrane assemblyhaving a central region positioned in overlying relationship to saidforward support; (c) providing a plurality of rigid contacts disposed onsaid central region each including a beam and a contacting portion, eachcontacting portion being arranged in suitable position for pressingengagement with a corresponding pad on said device, each beam beingelectrically connected to a corresponding flexible conductor thatextends into said central region; (d) providing a pressure controlmechanism including an elastomeric member interposed between eachcontact and said support; and (e) urging each beam, when thecorresponding contacting portion is placed into pressing engagement withthe respective pad, into tilting motion so as to move different portionsof each beam different distances relative to said support and so as todrive each contact in accordance with said tilting motion into lateralscrubbing movement across the corresponding pad, further includingresiliently opposing said tilting motion by means of said elastomericmember so as to enable each contact to recover from said tilting motionin a manner avoiding mechanical straining of each beam.
 13. The methodof claim 12 including urging a shorter one of said contacts into tiltingmotion at a later time than an immediately neighboring larger one ofsaid contacts.
 14. The method of claim 12 including providing a rearwardsupport tiltably coupled to said forward support and automaticallytilting said forward support relative to said rearward support toward aposition parallel to said device in response to pressing engagementbetween respective ones of said contacting portions and correspondingones of said pads.
 15. The method of claim 12 including providingcontacts in pairs and driving said contacts in each pair so that therespective lateral scrubbing movements are in opposite directions.